Which of these numbers is prime? ${57,\ 75,\ 78,\ 79,\ 81}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 57 are 1, 3, 19, and 57. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 79 are 1 and 79. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 79 is a prime number.